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That sounds like an error in the router hardware or software rather than a handset problem. The reboot cleared it. The VoIP call is being transmitted and received as digital internet data packets up to your router. The router has a section that "translates" this into a standard voice analogue phone signal, to suit standard handsets.

I've set up someone with a Siemen Gigaset 510ip. You can get extra handsets & chargers. They handle up to 4 VoIP services & 1 PSTN service & if you have multiple handsets, you can make/receive several concurrent calls

They have an "Exetel" configuration in a drop down list, which takes the guesswork out of the setup